1. Field of the Invention
This invention relates to a process for improving the properties of materials, and more particularly to a process for producing a metallic coating from a rapidly solidified metal.
2. Description of the Prior Art
Spray metallizing consists of heating a metal to a molten or semi-molten condition by passing it through a high temperature heat source, and depositing it in a finely divided form on a substrate. The molten or semi-molten particles flatten out on impacting the substrate and adhere to its surface. Subsequently deposited particles also flatten out, and adhere to those previously deposited, thus the structure of sprayed deposits is lamellar. The sprayed metal deposits resemble the derivative wire or powder chemically, but their physical properties, especially their microstructure, are quite different from those of the original wrought metal. Cohesion is achieved through mechanical and metallurgical bonding. After being sprayed, certain materials can be fused to form a dense and uniform coating that is metallurgically bonded to the substrate. Fused coatings usually are required for protecting the substrate material during service of high temperatures, in abrasive and corrosive environments, or for developing a surface of uniformly high hardness. For example, sprayed aluminum coatings on steel require heating to above 482.degree. C. to metallurgically bond the coating to the steel. Typically, the material may be subsequently heated at 732.degree. C. to 1093.degree. C. to provide a dense, uniform coating metallurgically bonded to the base metal.
Problems may arise due to the spray metallizing of aluminum coatings and subsequent diffusing of the aluminum spray coating from the formation of coarse aluminum-iron intermetallics dispersed within the deposited articles and at the coating/substrate interface. These intermetallics are very brittle and can degrade the mechanical properties of the component, for example, by forming a brittle layer between the components. Also, because the sprayed aluminum coating requires a thermal diffusing treatment, conditions may exist wherein the substrate material may not be properly heat treated. Problems may be encountered with welded aluminum coated steel parts. The alloying of the aluminum and iron can create a loss of ductility and lowering of corrosion resistance in the weld and heat-affected zone. Finally, because of the mismatch in the coefficient of thermal expansion between the sprayed aluminum coating and the substrate, the coating may degrade and spall off during high temperature exposure.